The art of click-type, adjustable torque wrenches is old and highly developed. Such wrenches typically include enlongate tubular lever arms with front and rear ends, manually engagable handles at the rear ends of the arms and work engaging heads at the front ends of the arms. The work engaging heads have elongate work coupling parts with axes normal to the longitudinal axes of the arms. The heads also have elongate rearwardly projecting levers that project freely rearwardly into and through the forward portions of the arms. The forward end portions of the levers are pivotally connected with the front end portions of the arms by pivot pins spaced rearward from and parallel with the axes of the heads and normal to the axes of the arms so that the levers can pivot laterally and into stopped engagement with the interior surfaces of the arms when the heads are drivingly coupled with work to be torqued and the arms are manually pivoted about the axes of the heads.
The rear ends of the levers have rearwardly disposed flat cam seats. The arms carry longitudinally shiftable plungers with flat, forwardly-disposed cam seats spaced from and opposing the cam seats on the levers. Cam blocks with flat, forward- and rearwardly-disposed cam faces are engaged between the noted cam seats and normally established flat seated engagement therewith. Compression springs are engaged in the arms rearward of the plungers to urge the plungers forward so that the cam faces of the cam blocks cooperate with their related cam seats to releasably hold the levers central and in axial alignment with the axes of the arms. Mechanisms are provided to adjust the axial biasing of and pressure exerted by the springs onto and through their related plungers, cam blocks and levers. The forces exerted by the springs determine the turning forces required to be applied onto and through the wrench structures to cause the levers to overcome the holding force afforded by the cam blocks between the plungers and levers and to allow the levers to pivot within the arms. When sufficient force is directed through and between the arms and levers to cause the levers to pivot in the arms, the cam blocks are caused to pivot between the plungers and levers, the plungers shift rearwardly against the resistance of the springs, and the levers pivot laterally and strike the inside surfaces of the arms, generating audible click sounds.
In the art of wrenches of the general character referred to above, there is another, more sophisticated and improved wrench structure which, in addition to the structure recited above, includes an elongate link with front and rear ends positioned within the arm between the lever and the plunger. The front end of the link is pivotally connected with the rear end of the lever by a pivot pin, on an axis parallel with and spaced rearward from the pivotal axis of the lever. The rear end of the link is formed with a rearwardly-disposed cam seat which normally establishes flat engagement with the front cam face of the cam block. The link is pivotally supported between its ends and within the arm by a fulcrum block which is carried by the link and engages the interior bearing surface of the arm. When the arm and lever pivot, that is, when the rear end of the lever is caused to move laterally to one or the other side of the arm, the front end of the link moves laterally within it, pivoting the link about the turning axis of the fulcrum block. Upon pivoting or turning of the link, the rear end thereof moves laterally to the other or opposite side of the arm. The rear end of the link cooperates with the cam block, plunger and spring in the same manner that the levers, cam block, plungers and springs of the first described form of torque wrenches cooperate.
The provision of the above noted link affords a mechanical advantage for the spring of the wrench whereby the spring need not be overworked and whereby a lighter, more durable spring can be employed. Further, by varying the longitudinal position of the fulcrum block on the link, wrenches for operating through many different ranges of operating forces can be made without making any other structural changes or modifications.
The above and last noted wrench structure is the subject matter of and is fully disclosed in U.S. Pat. No. 3,772,942 for "IMPROVED ADJUSTABLE TORQUE WRENCH," issued Nov. 20, 1973 and in U.S. Pat. No. 4,532,836 for "ADJUSTABLE FULCRUM FOR TORQUE WRENCHES," issued Aug. 6, 1985.
While the basic torque wrench structure disclosed in the above noted patents have proven to have many advantages over torque wrenches that do not include the noted link and fulcrum block, it is inherent in those wrenches that, as the levers pivot and cause the links and their fulcrum blocks to pivot in and relative to the lever arms, the fulcrum blocks, in addition to turning or pivoting, are also caused to reciprocate or shift axially short distances within the lever arms, as they pivot or turn therein. Still further, the applied forces or pressures exerted between the fulcrum blocks and the interior bearing surfaces of the arms are ordinarily quite great. As a result of the foregoing special factors inherent in the noted prior art wrench structure, it has been found that the bearing surfaces of the fulcrum block and their opposing bearing surfaces within the arms wear, at an excessive rate, with the result that worn depressions or grooves are formed in the bearing surfaces in the arms and the bearing points or edges of the fulcrum blocks wear into seated engagement in the worn depressions. As such wearing and seating of parts progresses, the ability of the fulcrum blocks to move longitudinally of the arms is impaired and effective, accurate operation of the wrenches is adversely affected. There are instances where such wearing and seating of parts has become so great that the wrenches have been rendered inoperative or their functioning has become so impaired as to render them undependable, if not useless.
In the above noted last-to-be-issued U.S. Pat. No. 4,532,836, wherein a metal bearing ring is engaged about and carried by the fulcrum block, a major reason for adopting and using the noted metal bearing ring was to provide a bearing part of a heat-treated alloy that had superior frictionally and wearing characteristics than could be economically and practically imparted into the fulcrum block itself. While the above proved to be effective with respect to wearing of the fulcrum bearing points for the links, it proved to have opposite and adverse effects with respect to the rate and extent of wear of the bearing surfaces within the arms. Since the special alloy bearing rings did not wear, the wearing forces exerted on the bearing surfaces in the arms remained concentrated and the high pressure turning and scuffing of the bearing ring parts wore deeper and more acute recesses in the bearing surfaces in the arms, at a faster rate.
Another notable shortcoming in the above noted wrenches provided by the prior art resides in the fact that the kind of motion that occurs at the fulcrum bearing points for the links tends to displace any lubricant that might be deposited to affect smooth operation and reduce wear. Also, when a lubricant is used which is less likely to be displaced, it inevitably collects, holds and concentrates foreign matter and the grindings from the worn metal parts, which foreign matter and grindings cause more rapid and severe wear.